Sexual and Apomictic Reproduction in Hieracium subgenus Pilosella Are Closely Interrelated Developmental Pathways

Abstract

Seed formation in flowering plants requires meiosis of the megaspore mother cell (MMC) inside the ovule, selection of a megaspore that undergoes mitosis to form an embryo sac, and double fertilization to initiate embryo and endosperm formation. During apomixis, or asexual seed formation, in Hieracium ovules, a somatic aposporous initial (AI) cell divides to form a structurally variable aposporous embryo sac and embryo. This entire process, including endosperm development, is fertilization independent. Introduction of reproductive tissue marker genes into sexual and apomictic Hieracium showed that AI cells do not express a MMC marker. Spatial and temporal gene expression patterns of other introduced genes were conserved commencing with the first nuclear division of the AI cell in apomicts and the mitotic initiation of embryo sac formation in sexual plants. Conservation in expression patterns also occurred during embryo and endosperm development, indicating that sexuality and apomixis are interrelated pathways that share regulatory components. The induction of a modified sexual reproduction program in AI cells may enable the manifestation of apomixis in Hieracium .

Genetic diversity and reproductive biology in ecotypes of the facultative apomict Hypericum perforatum L.

“…Thus, the sexual process is usually terminated soon after the aposporic initial cell differentiates in H. perforatum, as has been observed in most apomictic Hieracium (Koltunow et al, 1998(Koltunow et al, , 2000Bicknell et al, 2003). Our observations are consistent with the hypothesis that the aposporic initial might directly influence the sexual product degradation at this early stage (Tucker et al, 2003).…”

A survey of genes involved in Arachis stenosperma resistance to Meloidogyne arenaria race 1

“…The plasmids were linearised by digestion with Sac I or Sac II restriction enzymes and used as probes, after labelling with the Digoxigenin RNA in vitro transcription labelling kit (Roche, Indianapolis, IN, USA), according to the manufacturer's instructions. Root sections were pre-treated and hybridised with 2 ng of the labelled probe in 1 mL of hybridisation solution in a humid chamber at 42 C for 12 h, as described by Tucker et al (2003). Hybridisation sites were immunocytochemically detected using an anti-digoxigenin antibody conjugated to alkaline phosphatase (Roche), followed by incubation with NBT/BCIP buffer (nitro blue tetrazolium/5-bromo-4-chloro-3-indolyl-phosphate) for colorimetric detection of the phosphatase activity.…”

Sexual and asexual (apomictic) seed development in flowering plants: molecular, morphological and evolutionary relationships

“…Is this the same mechanism used by the functional megaspore? Molecular evidence indicates that apomictic and sexual reproduction are closely related pathways in Hieracium, since reproductive marker genes that are expressed in the MMC, embryo and endosperm show similar expression in apomictic and sexual plants (Tucker et al 2003). This suggests that sexual and apomictic pathways in Hieracium share common molecular regulatory features, indicating that they are not distinct pathways.…”

“…Morphologically, AI cells appear similar to both MMCs and functional megaspores, since they expand to a large size and contain a large nucleus and dense cytoplasm. In apomictic Hieracium, AI cells do not accumulate callose in their cell walls and unlike sexual Hieracium species fail to express MMC marker genes such as AtSPL:GUS and DMC1, and therefore are unlikely to share MMC identity (Tucker et al 2001(Tucker et al , 2003Okada et al 2007). However, this may not be the case in all aposporous species.…”